Cell membrane fusing liposomes for cytoplasmic delivery in brain endothelial cells

Direct cytoplasmic delivery is essential for susceptible molecules as proteins and some nucleic acids to improve their therapeutic efficacy in cells. Using liposomes for their delivery proved challenging due to known uptake by endocytosis followed by partial or complete lysosomal breakdown. Thus, ``fusogenic'' liposomes (FL) composed of the neutral lipid dioleoylphosphatidylethanolamine (DOPE) combined with the cationic lipid 1, 2-dioleoyl-3-trimethylammoniumpropane (DOTAP) were tested in different ratios for their cell membrane fusion ability and their cytoplasmic delivery was compared to ``pH-sensitive'' liposomes in murine brain endothelial cells (bEnd.3). They were loaded with cargos of different molecular sizes (calcein/ enhanced green fluorescent-protein (EGFP)/EGFP coding plasmid) and their intracellular delivery was quantitatively and qualitatively analyzed. FL composed of equimolar ratios of DOPE and DOTAP showed the most efficient cytoplasmic delivery of all cargos by fusing with the cell membranes within the first 15 min of addition. Their EGFP plasmid delivery to cells was quantified to be 58.2 +/- 9.5 % of the total EGFP load and calcein delivery was measured in buffer to be 64.1 +/- 4.0 % of the total calcein load, and reduced in blood to 26.1 +/- 0.6 %. Thus our tested FL allowed a fast and abundant cytoplasmic delivery of cargos independent of their molecular sizes while avoiding endocytosis, although they also underwent fast fusion with erythrocytes. Seemingly, these carriers could be used as a powerful delivery tool for in-vitro purposes.